Detect case-variant instruction filenames

[LE0-Lin]

Why

A scan of the pinned public RestSharp repository exposed a real false negative on case-sensitive systems: its root instruction file is named agents.md, so the scanner previously ignored it completely.

What changed

• discover supported instruction filenames case-insensitively
• preserve AGENTS-family scope and precedence behavior for case variants
• prefer the documented spelling if case variants coexist on a case-sensitive filesystem
• add RestSharp at a pinned commit as a reviewed real-repository regression case
• allow targeted benchmark reproduction with --repository OWNER/REPO

Evidence

At restsharp/RestSharp@64ee129, the scanner now discovers three instruction files and reports:

• context-large in agents.md
• rm-rf at agents.md:661
• B 84 overall

No external repository code is executed by the benchmark.

Verification

• py -3.12 -m unittest discover -s tests -v — 134 passed, 1 skipped on Windows (case-sensitive coexistence test)
• py -3.12 -m agent_config_score.cli examples/demo — expected F 56 demo result
• py -3.12 scripts/run_real_world_benchmark.py --repository restsharp/RestSharp — matched reviewed expectation
• full-corpus replay was attempted three times but GitHub clone downloads were interrupted by transient TLS/RPC errors
• git diff --check